Lipoprotein receptors have essential functions in the control of plasma lipid levels and are critically important for protection of the arteries from atherosclerosis and coronary artery disease. The low-density lipoprotein (LDL) receptor gene family represents one class of such receptors that bind the lipid transport protein Apolipoprotein E. Three isoforms of ApoE exist in the human population, E2, E3 and E4. ApoE4 is genetically associated with increased risk for two major, but clinically very different diseases that are prevalent in our Western populations: Atherosclerosis and Alzheimer's disease (AD). The molecular mechanisms through which ApoE4 genotype predisposes to higher circulating LDL levels on one hand, and to increased neuronal vulnerability and accelerated degeneration in AD on the other, are poorly understood. Understanding the role of ApoE in these disease processes, however, is of rapidly increasing socioeconomic importance in our Western societies, where the continuing increase in average life-expectancy has led to an epidemic of age- related vascular and neurological debilities. The purpose of this proposal is to investigate the molecular and cellular mechanisms by which ApoE promotes disease onset and progression, and to determine the role of the ApoE receptors Vldlr and Apoer2 in the process. Both proteins are not only receptors for ApoE, but also for Reelin, an indispensable neurodevelopmental signaling protein that further controls synaptic plasticity by activating Vldlr and Apoer2 in the adult brain. ApoE is produced abundantly by glial cells in the CNS, where we hypothesize that it interferes with Apoer2 and Vldlr functions, thereby contributing to the pathogenesis of AD. Intriguingly, ApoE, Reelin, Apoer2 and Vldlr also cross paths in the periphery at the vascular wall. Reelin is secreted by the liver and circulates in blood in concentrations similar to those present in the CNS. Its role in the blood is unknown, but the Reelin receptors Apoer2 and Vldlr have protective roles in the maintenance of the vascular wall. This proposal explores the function of Reelin at the vessel wall and investigates molecular mechanisms through which ApoE interferes in an isoform-specific manner with ApoE receptor functions and Reelin signaling in the vascular wall and in neurons. The proposed work involves the analysis of the vascular physiology of conditional Reelin knockout mice, the effect of naturally occurring and mutant ApoE isoforms on subcellular ApoE receptor trafficking and recycling, and detailed quantitative measurements of Reelin signaling functions in cells. The results from these studies will contribute a conceptually novel mechanistic basis to our understanding of the pathogenesis of atherosclerosis, as well as ApoE-mediated neurodegenerative processes.